This invention relates to a gas spring and, particularly to a gas spring of the kind including a generally vertical cylinder having a closed lower end, a piston working in the cylinder, a piston rod secured to the piston rod and extending out of the cylinder through the upper end thereof, and pressurized gas enclosed in the cylinder. A rod guide is usually mounted on the upper end of the cylinder for guiding the sliding movement of the piston rod and mounting a seal which slidingly engages with the piston rod and seals the interior of the cylinder from the outside.
Since the sliding movement between the piston rod and the seal is performed in a dry condition, the frictional resistance is large and the seal is apt to be damaged.
For eliminating the shortcomings aforementioned, there has been proposed an arrangement to provide an oil chamber in the upper portion of the cylinder as shown, e.g. U.S. Pat. No. 4,030,716. A portion of the piston rod contacts with the oil in the oil chamber, thus the piston rod contacts with the seal in a wet condition, thereby improving the sliding characteristics. However, it is necessary to fill into the cylinder the high pressure gas and the oil separately and the filling operations are difficult and time consuming.